The research in this grant investigates the function of the E2F transcription factor. E2F is a heterodimeric factor composed of a polypeptide encoded by DP-1 (or DP-1-related genes) and a polypeptide encoded by E2F-1 (or E2F-1-related genes). Studies in mammalian cells show that E2F plays an important role during progression rom G1 to S phase of the cell cycle. E2F binding sites are found inn the promoters of several genes whose expression is linked to cell proliferations. Analysis of several of these promoters suggest that the E2F sites confer transcription activation at specific times in the cell cycle. In normal cells E2F activity is regulated by the phasic activation of cyclin dependent kinases (cdk's) and provides a connection between cell cycle progression and specific transcriptional events. In tissue culture cells the deregulation of E2F has interesting effects. The overexpression of the E2F-1 gene has been found to drive quiescent cells into S-phase and prevent cycling cells from exiting the cell cycle. The long-term outcome of this apparently depends on the cell types used; high level expression of E2F-1 has been shown to transform some immortalized cell lines and to cause apoptotic cell death in others. We propose to study the in vivo function and regulation of E2F activity. Our goal is to determine the role of E2F in cell proliferation and differentiation during normal development and identify features important for the regulation of this function. These studies will be carried out in Drosophila using the dE2F and dDP homologs that we have recently cloned and the series of dE2F mutants that we have recently identified. The extensive families of mammalian E2F and DP genes appear to be functionally replaced in Drosophila by one gene of each type. This facilitates experiments into the function of E2F that are not possible in mammalian cells. The objectives of this proposal are; first to determine when and where dE2F and dDP are expressed during development and to compare this with the appearance of E2F transcriptional activity; second, to investigate the biological role of E2F by studying the consequences of loss of dE2F function throughout development; third, to perform a structure/function analysis of dE2F gene by testing the activities of dE2F mutants in animals deficient for E2F; fourth, to investigate the regulation of dE2F activity by analyzing dE2F interactions with functionally related genes.